TUN inbound: Block DNS and IPv6 leaks outside the TUN on Windows; Add strictRoute

Windows sends name queries to the DNS servers of all interfaces, and a
resolver on the local network (e.g. 192.168.1.1 from DHCP) is reached
through its more specific LAN route instead of the TUN, so DNS leaks
past it. IPv6 bypasses a TUN that cannot carry it.

With autoSystemRoutingTable set, the Windows TUN now adds Windows
Filtering Platform filters, all in one transaction and in a dynamic
session, so that they are removed when Xray exits, even if it crashes:
- DNS (port 53) only goes through the TUN, in both directions: its local
  address, and the interface it leaves or arrives by, must be the TUN's.
- IPv6 is blocked in both directions when the TUN has no IPv6 address or
  no IPv6 route, except loopback, neighbor and multicast listener
  discovery, and DHCPv6.
- Xray's own traffic is exempt: its connections out with a hard permit,
  which Windows Firewall rules do not override (like sing-box's
  strict_route), connections to its inbounds with an ordinary one.
If the filters cannot be added, the TUN does not start on Windows 10 and
later (only a warning on 7/8). The new `strictRoute` option (true by
default) turns them off.

Also on Windows:
- A warning for `dns` servers outside gateway and autoSystemRoutingTable,
  as queries to them cannot go through the TUN and are blocked.
- While DNS is restricted and autoOutboundsInterface is in use, Xray
  resolves the names it would ask Windows for itself (Go's resolver on
  its own sockets). Those lookups and the `localhost` DNS server skip the
  TUN's DNS servers, unless another interface uses them too, instead of
  looping back into the TUN.
- The DNS cache is flushed when the TUN starts and stops, and DNS
  registration is turned off on the TUN (through netsh before Windows 10
  1809).
- Close no longer panics when registering the route or interface change
  callbacks failed.
The README's Windows section describes all of it.

Tested on Windows 11, elevated, amd64 and 386: the filters, DNS arriving
through a real Wintun adapter and blocked outside it, the IPv6 block,
Windows Firewall rules, and a real Xray run. Windows 7/8 and Windows 10
before 1809 are untested.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

proxy/tun/config.pb.go

@@ -33,6 +33,7 @@ type Config struct {
 	AutoOutboundsInterface string                 `protobuf:"bytes,7,opt,name=auto_outbounds_interface,json=autoOutboundsInterface,proto3" json:"auto_outbounds_interface,omitempty"`
 	Desc                   string                 `protobuf:"bytes,8,opt,name=desc,proto3" json:"desc,omitempty"`
 	AutoSystemDns          bool                   `protobuf:"varint,9,opt,name=auto_system_dns,json=autoSystemDns,proto3" json:"auto_system_dns,omitempty"`
+	StrictRoute            *bool                  `protobuf:"varint,10,opt,name=strict_route,json=strictRoute,proto3,oneof" json:"strict_route,omitempty"`
 	unknownFields          protoimpl.UnknownFields
 	sizeCache              protoimpl.SizeCache
 }
@@ -130,11 +131,18 @@ func (x *Config) GetAutoSystemDns() bool {
 	return false
 }
 
+func (x *Config) GetStrictRoute() bool {
+	if x != nil && x.StrictRoute != nil {
+		return *x.StrictRoute
+	}
+	return false
+}
+
 var File_proxy_tun_config_proto protoreflect.FileDescriptor
 
 const file_proxy_tun_config_proto_rawDesc = "" +
 	"\n" +
-	"\x16proxy/tun/config.proto\x12\x0exray.proxy.tun\"\xaa\x02\n" +
+	"\x16proxy/tun/config.proto\x12\x0exray.proxy.tun\"\xe3\x02\n" +
 	"\x06Config\x12\x12\n" +
 	"\x04name\x18\x01 \x01(\tR\x04name\x12\x10\n" +
 	"\x03MTU\x18\x02 \x01(\rR\x03MTU\x12\x18\n" +
@@ -145,7 +153,10 @@ const file_proxy_tun_config_proto_rawDesc = "" +
 	"\x19auto_system_routing_table\x18\x06 \x03(\tR\x16autoSystemRoutingTable\x128\n" +
 	"\x18auto_outbounds_interface\x18\a \x01(\tR\x16autoOutboundsInterface\x12\x12\n" +
 	"\x04desc\x18\b \x01(\tR\x04desc\x12&\n" +
-	"\x0fauto_system_dns\x18\t \x01(\bR\rautoSystemDnsBL\n" +
+	"\x0fauto_system_dns\x18\t \x01(\bR\rautoSystemDns\x12&\n" +
+	"\fstrict_route\x18\n" +
+	" \x01(\bH\x00R\vstrictRoute\x88\x01\x01B\x0f\n" +
+	"\r_strict_routeBL\n" +
 	"\x12com.xray.proxy.tunP\x01Z#github.com/xtls/xray-core/proxy/tun\xaa\x02\x0eXray.Proxy.Tunb\x06proto3"
 
 var (
@@ -177,6 +188,7 @@ func file_proxy_tun_config_proto_init() {
 	if File_proxy_tun_config_proto != nil {
 		return
 	}
+	file_proxy_tun_config_proto_msgTypes[0].OneofWrappers = []any{}
 	type x struct{}
 	out := protoimpl.TypeBuilder{
 		File: protoimpl.DescBuilder{